Pinnatins A−E: Marine Diterpenes of the Rare Gersolane Class Derived from a Photochemically Induced Rearrangement of a Conjugated 2,5-Bridged Furanocembrane Precursor

Ring contraction in synthesis of functionalized carbocycles

Carbocycles are a key and widely present structural motif in organic compounds. The construction of structurally intriguing carbocycles, such as highly-strained fused rings, spirocycles or highly-functionalized carbocycles with congested stereocenters, remains challenging in organic chemistry. Cyclopropanes, cyclobutanes and cyclopentanes within such carbocycles can be synthesized through ring contraction. These ring contractions involve re-arrangement of and/or small molecule extrusion from a parental ring, which is either a carbocycle or a heterocycle of larger size. This review provides an overview of synthetic methods for ring contractions to form cyclopropanes, cyclobutanes and cyclopentanes en route to structurally intriguing carbocycles.

Role of Macrocyclic Conformational Steering in a Kinetic Route toward Bielschowskysin

Macrocyclic furanobutenolide-derived cembranoids (FBCs) are the biosynthetic precursors to a wide variety of highly congested and oxygenated polycyclic (nor)diterpenes (e.g. plumarellide, verrillin, and bielschowskysin). These architecturally complex metabolites are thought to originate from site-selective oxidation of the macrocycle backbone and a series of intricate transannular reactions. Yet the development of a common biomimetic route has been hampered by a lack of synthetic methods for the pivotal furan dearomatization in a regio- and stereoselective manner. To address these shortcomings, a concise strategy of epoxidation followed by a kinetically controlled furan dearomatization is reported. The surprising switch of facial α:β-discrimination observed in the epoxidation of the most strained E-acerosolide versus E-deoxypukalide and E-bipinnatin J derived macrocycles has been rationalized by the variation of the 3D conformational landscape between macrocyclic scaffolds. A careful conformational analysis of these macrocycles by VT-NMR and NOESY experiments at low temperature was supported by DFT calculations to characterize these equilibrating macrocyclic conformers. The shift in conformational topology associated with a swing of the butenolide ring in E-deoxypukalide is in general agreement with the reversal of β-selectivity observed in the epoxidation. We also describe the downstream functionalization of FBC-macrocycles and how the C-7 epoxide configuration is retentively translated to the C-3 stereogenicity in dearomatized products under kinetic control to secure the requisite 3S,7S,8S configurations for the bielschowskysin synthesis. Unlike previously speculated, our results suggest that the most strained FBC-macrocycles bearing a E-(Δ^7,8)-alkene moiety may stand as the true biosynthetic precursors to bielschowskysin and several other polycyclic natural products of this class.

Recent Advances of Total Syntheses of Diterpenoids Starting from Carvone

This review focuses on total syntheses of diterpenoids starting from carvone (1) since 2006 in the alphabetical order of compound names.

Light-mediated total synthesis of 17-deoxyprovidencin

An asymmetric synthesis of the diterpenoid 17-deoxyprovidencin is described. Key steps include an aldol addition, a base-catalyzed Wipf-type furan formation, a Z-selective ring-closing metathesis for macrocyclization, a photochemical E/Z isomerization to a highly strained and conformationally restricted ring system, and the stereoselective formation of two epoxides on the ring.

Multicomponent Tandem Synthesis of Oxospirobicyclic Butenolidobarbiturates

Novel oxospirobicyclic γ-butenolides containing barbiturate moiety have been synthesised by the Michael-addition of primary amines to dialkyl acetylenedicarboxylates followed by aldol-like reaction with alloxan derivatives, and then γ-lactonisation. Subsequent 1,4-addition of water and elimination of the amine group completes the sequence. This tandem reaction sequence represents a rapid and unprecedented route to the described biologically interesting molecules.

Diterpenes from gorgonian corals

Covering: 1995 to April 2008. Gorgonian corals continue to provide a wealth of novel structures, many of which exhibit potentially useful biological activity. Notably, the families Briareidae, Gorgoniidae and Plexauridae have been demonstrated to contain a wide variety of natural products including steroids, acetogenins, sesquiterpenes and diterpenes. The most common of the gorgonian natural products are the diterpenes, and the intent of this review is to describe such compounds isolated from gorgonian corals, with a focus on the structures of new compounds as well as their biological activity. There have been developments improving our understanding of the biosynthetic origin of selected diterpenes, and these will also be discussed. This review describes 602 new compounds from 177 articles.

Facile Synthesis of β-Organotellurobutenolides via Electrophilic Tellurolactonization of α-Allenoic Acids

We report a convenient and highly efficient method for the synthesis of beta-organotellurobutenolides by the aryltellurenyl halides-induced electrophilic tellurolactonization of alpha-allenoic acids under mild conditions. The resulting beta-organotellurobutenolides can be utilized as precursors for versatile butenolide derivatives through a substitution reaction with organocuprate reagent or Pd/Cu(I)-catalyzed cross-coupling with terminal alkyne.